1. Field of the Invention
The present invention concerns a method and equipment including a processing tool for the processing of carbon bodies, such as the formation of holes in carbon bodies, in particular nipple holes in anodes for use in electrolytic cells for the production of aluminum.
2. Description of Related Art
In modern electrolysis plants for the production of aluminum, anodes are used which comprise a prebaked or calcined carbon body fastened to an anode hanger. The anode hanger usually consists of an upper part, an anode rod made of aluminum and a lower part comprising a yoke having steel nipples which extend downwards.
The carbon body is fastened to the steel part of the anode hanger using a cast iron or monolithic lining material joint. More specifically, the joint between the carbon body and the steel part is formed by the nipples in the steel part that are first arranged so that they extend downwards into holes in the carbon body and then the holes are filled with molten cast iron or having monolithic lining material pressed into them.
The holes in the carbon body are formed in the traditional manner during the forming of the carbon bodies, the so-called "green carbon", before the calcination process (prebaking process).
This results in the following disadvantages:
The green carbon must be stacked in a special way in the calcination furnaces to avoid creep and deformation of the holes. This special stacking results in additional work and reduced calcination capacity, i.e. reduced productivity.
However, even if the precautions mentioned are taken, some holes will still be deformed and, consequently, some carbon bodies will be unusable. The proportion of rejects may be as much as 10% or more in some cases.
Another problem in connection with the calcination of carbon bodies with preformed holes is that the packing material can burn fast to the walls of the holes.
Other disadvantages are that preformed nipple holes are associated with large density gradients and many small cracks in the carbon form body around the nipple hole. The cracks will result in a high electrical contact resistance, which is not very desirable during the electrolysis process as the increase in resistance can lead to an increase in the temperature in the joint between the anode and the nipple.
Another disadvantage of preforming the nipple holes is that there are restrictions to the geometry which can be formed with this technique as they are usually formed using projections in the vibration mold, which must be provided with a certain taper so that they can be withdrawn from the body when the stamping operation has finished. If undercut or conical nipple holes are to be formed, which will gradually become more common, the hole must be processed further.
Several solutions are proposed for cleaning the nipple holes of an anode after calcination. For example, FR 2 590 911 shows a two-part cleaning tool for nipple holes in which a first, outer part is designed to clean grooves in the walls of the hole while a second part is designed to smooth the peripheral surface and base of the hole. The latter part is driven by a rotating shaft while the first part is arranged coaxially and supported freely in relation to the shaft. This equipment, which, among other things, is designed to clean grooves in the walls of the hole by means of axial motion, will be limited to cleaning preformed holes.
EP 0 764 728 A 1 shows equipment for processing three nipple holes in one anode. The equipment comprises a centering device for centering an anode with performed holes, a liftable/lowerable support for the anode and three downward-facing spindles each with its own milling head for processing the anode's holes.
In the above publication, it is suggested that it would be conceivable to form holes in anode blocks without preformed holes with the processing tool and, moreover, that this can be done with sufficient precision. However, no indication of what such a tool would look like is given.